1. Field of the Invention
This invention relates to a textile fabric suitable for use as a fire-blocking sheet in aircraft seats and more particularly concerns such a fabric comprising several layers of hydraulically needled batts of aramid staple fibers.
2. Description of the Prior Art
Aircraft seat cushions now in use typically comprise a core of flame-retardant polyurethane foam and a woven upholstery outer covering (also called a "dress fabric"). The materials and construction of these seat cushions are designed to prevent an accidental fire from occurring if a lighted cigarette were to come into contact with the seat. However, in a full-scale cabin fire, the outer upholstery cover commonly used on seat cushions rapidly breaks down and the polyurethane core becomes involved in the fire, producing large volumes of potentially lethal smoke, combustible gases and toxic gases. For aircraft safety, the Federal Aviation Administration (FAA) requires in the United States, that the foam core of an aircraft seat cushion must be fully encapsulated by a fire-blocking sheet which delays the onset of ignition and retards involvement of the core in the fire. For some aircraft, the seat cushion must also be buoyant for use as an emergency life preserver in the event of a crash landing in water. In addition to meeting FAA requirements, the fire blocking sheet preferably should be lightweight as practicable so that aircraft fuel consumption might be minimized.
Expanded papers made from commingled mixtures of aramid fibrids and aramid fibers, including blends of fibers of poly(m-phenylene isophthalamide) and poly(p-phenylene terephthalamide), have been disclosed in Nollen et al, European Patent Application Publication No. 0 128 712 as being useful for fire blocking sheets. However, these expanded papers do not meet the standards imposed by commercial airlines with respect to puncture resistance. Such fire blocking sheets usually would be punctured by high-heeled shoes, if a person wearing such shoes were to stand on the seat.
Woven and nonwoven fabrics of aramid fibers and blends thereof are now widely used in protective clothing worn by aviators, race drivers, and others who risk exposure to fires or an intense thermal flux.
Burckel, U.S. Pat. No. 4,198,494 discloses fiber blends which comprise, for example, at least 15% poly(m-phenylene isophthalamide) (MPD-I) fibers and 3 to 20% poly(p-phenylene terephthalamide) (PPD-T) fibers. The MPD-I fibers meld or fuse when exposed to intense thermal flux and the PPD-T fibers have high flame strength when in fabric form. Clothing made with such fibers is resistant to fabric break-open when exposed to flame or intense thermal flux.
Behnke, U.S. Pat. No. 4,120,914 discloses blends of 45-55 wt % MPD-I fibers with 45-55% wt % PPD-T fibers which, in fabric form, exhibit good resistance to shrinkage at high temperature and are correspondingly useful for protective clothing.
Research Disclosure, Oct. 1982, Item 22215 discloses that spunlaced nonwoven fabrics made of the MPD-I/PPD-T fiber blends provide even more thermal protection than woven fabrics of the same basis weight.
In view of the foregoing disclosures, it might have been hoped that woven or nonwoven fabrics of MPD-I/PPD-T blends would be effective as fire blocking fabrics for aircraft seat cushions. However, the fabrics of MPD-I/PPD-T blends that have been available hitherto have failed to meet the FAA requirements.